This invention relates to a device for containing the force of an explosion, particularly, an electrical explosion.
In underground tunnels carrying high voltage cables the occurrence of explosions at the circuit connectors is relatively commmon. This can pose a major safety hazard to workmen in the tunnels and cause secondary damage to nearby cables. What usually happens is that the junction of two conductors within a circuit connector overheats and causes the oil filled connector to explode with considerable force. The explosion is accompanied by a rapid arc discharge to ground. The rapid electrical discharge has a similar effect to a lightning bolt.
Currently, there is no effective device for affording protection to personnel and equipment in the tunnels. An object of the present invention is to provide such a device that also has general application to other situations where explosion protection is required.
According to the present invention there is provided a device for containing the force of an explosion, comprising a blanket or curtain of flexible material that can be draped around a danger zone, said blanket or curtain comprising a plurality of layers of fabric, each layer having a tight, balanced weave independently woven from a tough, non-combustible yarn of at least 1,000 denier, said fabric being substantially uninterrupted by stitching in a central zone intended to take the main force of the explosion, and said fabrics being bound together at their edges with a border of tough material extending around the periphery of the blanket or curtain, and attachment means disposed across said border.
The blanket or curtain, which is normally made of Kevlar (a trademark for poly(p-phenylene terephthalamide)), can be designed as a permanent fixture to be hung in the tunnel or as a portable device to be folded up and carried about in a truck. Eyelets are preferrably provided around the perimeter of the blanket to enable it to be suspended from hooks attached to racks in the tunnel. As permanent fixtures, the curtains can be forty or more meters wide, whereas when used as supportable blankets they are usually about two or three meters wide.
When the blanket is placed over a circuit connector and an explosion occurs, the blanket or curtain can provide effective protection up to 15 kv when the fabric is made of Kevlar (a trademark for poly(p-phenylene terephthalamide)). For this purpose at least three layers of fabric should be employed. The first layer will usually become completely charred, but the remaining layers will provide effective protection both against the force of the explosion and the subsequent electrical discharge. When used as a permanent fixture, the curtain will prevent secondary damage to nearby cables or workmen in the tunnel. As a portable device, it will provide effective protection to workmen in manholes or underground tunnels. The workmen merely hang or drape the blanket around the danger zone.
In a preferred embodiment for use in a damp environment, the layers of fabric are coated on their inside surfaces with a mixture of Hypalon (a trademark for chlorosulphanated polyethylene) and a suitable thermoplastic such as polyvinyl chloride, nylons, fluorocarbons, linear polyethylene, polyurethane prepolymer, polystyrene, polypropylene, and cellulosic and acrylic resins.
The outer surfaces of the blanket are coated with a water repellent agent such as Zepel (a trademark for a fluorocarbon textile finish). The layers of fabric are cut together with a hot knife and the thermoplastic in the coating seals the edges together prior to the application of the tough border so as to prevent the ingress of moisture between the fabric layers. Also, when the fabrics are stitched together, the friction from the sewing needle momentarily softens the thermoplastic, which recools around the thread and thus avoids the ingress of moisture through the needle holes. A blanket or curtain constructed in this manner can provide effective protection against electrical explosions even when the curtain is partly immersed in water.
The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings.